1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling a cell addition for dual connectivity and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3GPP as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and for communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint (COMP) transmission/reception, UL multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
Small cells controlled by low-power base stations (e.g., low power NBs/eNBs) are considered to solve fast-growing mobile traffic. The small cells can be deployed in hot spots for both indoor and outdoor scenarios. A low-power base station generally means a base station with a transmission power lower than that of a macro cell base station (e.g., normal NB/eNB). For example, a pico cell base station and a femto cell base station are usually considered as low-power base stations. A UE may simultaneously communicate with a macro cell base station and a low-power base station, to realize dual connectivity. In this situation, the UE can perform transmissions and/or receptions (e.g., of data, packets, messages and/or control information) via both the macro cell base station and the low-power base station. The data throughput increases due to simultaneous data transmissions from/to the macro base station, when the dual connectivity is operated. However, it is unknown how to configure a cell of the low-power base station to the UE to enable the dual connectivity, i.e., adding a cell of the low-power base station while the UE has connected to the macro cell base station. In addition, it is also unknown how to assign a cell radio network temporary identifier (C-RNTI) of the cell of the low-power base station to the UE when realizing the dual connectivity.
Thus, how to add the cell to enable the dual connectivity and to assign the C-RNTI are important problems to be solved.